During embryonic brain development, patterned connectivity, neuronal positioning, and cooperative signaling are established for sensory systems. This temporal window can be utilized to investigate how features of individual synapses relate to function and how neuronal ensembles within and between separate nuclei collectively support sensation. Unfortunately examination of sensory systems during this time period has, for the most part, not been systematically undertaken. The proposed investigations focus upon early development of a conspicuous synaptic connection of the auditory brainstem, the Calyx of Held terminals at the medial nucleus of the trapazoid body (MNTB). Although many details involving neuronal and synaptic physiology at mature connections of the central auditory system have been determined, information regarding embryonic MNTB nucleus formation, initial synaptic connectivity phases, formation of patterning as it relates to auditory function, and onset of functional synaptic signaling at these neurons remains obscure. Through experimental strategies that combine optical recording of morphologic and functional synaptic features with physiologic data from whole-cell recording, initial functional connectivity of this auditory relay will be characterized. These studies will provide fundamental information regarding the nature and timing of early connections that later transform into large, mature nerve terminals. Detailed confocal imaging observations over the developmental period of MNTB formation (embryonic day 14 - postnatal day 4) will identify characteristics and structural patterns of both pre-calyx axonal terminals and postsynaptic MNTB principal neurons. Analysis and quantification of morphologic and functional features of early connections will also yield clues regarding mechanisms that guide MNTB morphogenesis, synaptic formation, and functional input patterning. Finally, the proposed experiments will begin to examine the role that the inhibitory signaling of the GABAergic system plays in guiding the early development and functional patterning of the MNTB. PUBLIC HEALTH RELEVANCE: The proposed experiments will provide a fundamental understanding of how brain connections of a particular auditory system circuit first form at junctions known as synapses, and how these many connections are precisely coordinated during development. The long term goal of these and future studies is to gain a detailed understanding of the process and mechanisms involved in auditory synapse formation to understand how neurons organize into groups and select partners for cooperative signaling that supports normal hearing function. Information gained in this study could be of direct relevance for understanding certain sensory disturbances associated with autism.